When laying pipe in a ditch, the bottom of the ditch is never truly level or straight. A ditch will follow the terrain, and the terrain is rarely on a straight line or level. There are rises and dips or fails. Thus, when joining pipe sections, a measure of flexibility at the connected areas or joints is highly desirable without the loss of the seals for the connected parts.
Moreover, when laying and connecting pipe sections of large diameter and of substantial weight, it is difficult to rotate the pipe sections in order to connect them to one another.